There is no specific time frame. In a TBI, Stoke and many other types of brain insult there is permanent damage to the brain, due to lack of blood flow and thus anoxic and hypoxic conditions. But there is no distinct line where the damage ends and viable brain tissue begins. Rather there is an area around the necrotic brain tissue that is not fully functioning but is also not necrotic. This area is called the penumbra and consists of neurons that have enough oxygen to stay alive but not enough to reach full capacity... it is said to be in a plastic state. Two functions of Hyperbaric Hyperoxia are an increase in stem cell mobilization and angiogenesis or the production of new blood vessels. The body innately knows where these areas of low oxygen are and intrinsically builds new blood vessels to and in them thus allowing salvage and recovery of parts of the penumbra. These neurons become available permanently and recovery can be significant even years after the injury or insult. Take a look at this study that shows significant recovery for stroke victims months and sometimes years after the event. It is very interesting because 100% saw improvements in both quality of life and motor skills. Why is this ignored by the drug based medical establishment?http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3546039/pdf/pone.0053716.pdf